Thermal Expansion Tanks and Mixing Valves for Water Heaters in Port Orchard and the West Sound

What thermal expansion does inside a closed plumbing system

Water expands when it is heated. A fifty-gallon tank warming from fifty degrees to a hundred and twenty gains about three quarters of a gallon in volume. In an old house with a plain meter and no check valve, that extra volume simply pushed back out into the street main and nobody noticed. Modern water systems are different. Pressure reducing valves, backflow preventers and the check valves built into newer meters all stop water from flowing backward, which turns your house into a closed container.

In a closed container that extra three quarters of a gallon has nowhere to go, so pressure climbs sharply every time the heater runs. House pressure that reads sixty at the hose bib can spike well above a hundred and fifty during a heating cycle. The temperature and pressure relief valve on the heater is set to open at one fifty, and it does its job: it drips. The toilet fill valves hiss, the faucet cartridges wear out, the washing machine hoses bulge, and the tank itself is being flexed thousands of times a year.

An expansion tank is a small steel vessel with a rubber bladder and an air cushion on one side. When the water expands, it compresses the air instead of straining the pipes. Washington's plumbing code has required one on any heater in a closed system for years, which is why home inspectors around Port Orchard and Gig Harbor flag their absence so often.

Signs the expansion tank is missing or has failed

The clearest sign is the relief valve. A T&P valve that drips only after a shower, or leaves a puddle in the pan each morning, is usually responding to thermal expansion rather than a bad valve. Replace the valve without addressing the pressure and the new one will drip too. We test by watching a gauge on the drain valve through a full heating cycle, which shows the spike in real time.

An expansion tank that is present but dead is almost as common. The bladder tears or the air charge leaks away, the tank fills completely with water, and it stops doing anything. Tap the tank: a working one sounds hollow on top and solid on the bottom. A waterlogged one sounds solid throughout and is noticeably heavy. Pressing the Schrader valve on a waterlogged tank squirts water instead of air.

The other failure is mechanical. A tank that was screwed onto a horizontal tee with no support eventually works the threads loose or cracks the fitting, and the result is a leak at the top of the heater that gets blamed on the heater. We mount tanks with a strap or bracket and, where the space allows, vertically with the connection on the bottom so the bladder does not sit under the tank's weight.

• Relief valve drips after the heater runs, not continuously
• A thump or bang in the pipes when the heater shuts off
• Pressure gauge on a hose bib reads over a hundred after a heating cycle
• The expansion tank is heavy and sounds solid when tapped
• Water, not air, comes out of the tank's Schrader valve
• Home inspection report lists a missing or unsupported expansion tank

Sizing and setting an expansion tank correctly

The small two-gallon tank sold at the hardware store is right for a forty or fifty-gallon heater at moderate pressure. A seventy-five or eighty-gallon tank, a high-recovery commercial heater, or a house running on the high side of pressure needs a larger vessel. We check the heater size and the actual static pressure and pick the tank from the manufacturer's chart rather than guessing.

The air charge matters more than most people realize. Out of the box the tank is pre-charged to a generic number that rarely matches the house. If the charge is lower than the house pressure, the bladder is already compressed before the heater fires and the tank has no room to absorb anything. We measure static pressure at the house, set the tank's air side to match with a hand pump and gauge, and only then screw it on. This takes five extra minutes and it is the step that gets skipped.

On homes with a pressure reducing valve, which is most of the newer construction in Silverdale, Gig Harbor North and Puyallup, we also confirm the PRV is holding its setting. A PRV that has drifted high makes every expansion problem worse and shortens the life of the new tank. Well homes with a pressure tank already have a cushion in the system and often do not need a separate expansion tank at all; we tell you when that is the case.

Thermostatic mixing valves and tank boosters

A thermostatic mixing valve sits on the water heater outlet and blends hot water with a little cold so the water leaving the heater holds a set temperature no matter how hot the tank is. That lets you do something useful: turn the tank up. Water stored at a hundred and forty degrees or higher stops Legionella bacteria from growing and, because it is hotter, each gallon stretches further when mixed down at the shower. A fifty-gallon tank with a mixing valve delivers roughly what a sixty-five or seventy-gallon tank would without one.

That is why they are sometimes sold as tank boosters. For a family of five in a Bremerton rambler that keeps running out of hot water, a mixing valve on the existing heater is often a better answer than a bigger tank, and it is a far smaller job. We dial it in with a thermometer at the valve outlet and again at the farthest fixture.

Mixing valves also solve a safety problem. A hundred and forty degree water scalds skin in seconds. Anywhere there are small children, elderly family members, or a tenant you cannot supervise, a mixing valve on the heater is the simplest protection you can buy, and it is required on new installations by most of the jurisdictions we work in.

Doing both at once during a water heater replacement

Every water heater Washington Plumbing & Drain installs leaves with a properly sized and charged expansion tank, a mixing valve where the home needs one, a drain pan, seismic straps and a new T&P valve. The photos on this page show what that package looks like in a closet, a garage and under a staircase. If another company quoted you a heater swap without these, ask why.

When we retrofit these on an existing heater, the job is typically an hour or two. We shut the water off, drain a little pressure, cut in a tee for the expansion tank on the cold side, install the mixing valve on the hot outlet with unions so it can be serviced, and pressure test everything before relighting. You get a flat price for the whole thing before we pick up a wrench, and the parts carry our warranty along with the manufacturer's.